An image sensor is a component that converts a visible image to an electric signal in a camera (e.g., a digital camera). The image sensors can be divided into two categories: CMOS (complimentary metal oxide semiconductor) or CCD (charge coupled device) sensors. CMOS sensors are used commonly in mobile phones due to their higher degree of integration, lower price and lower power consumption.
One of the biggest problems in a CMOS sensor is its property called a rolling shutter. If a sensor has the rolling shutter, it is exposed line by line, instead of globally. Since each line is exposed at a slightly different time moment, images of moving targets seem to bend. This bending is called a motion skew.
The motion skew can be avoided by using a mechanical shutter, but, when a mechanical shutter is used, the exposure time cannot be set very accurately. This is because capturing an image with a mechanical shutter requires mechanical movement, which is never perfectly accurate. The exposure time of the same mechanical shutter can vary, e.g., ±0.2 ms. So if the required exposure time is 1 ms, the actual exposure time with the mechanical shutter is between 0.8 ms and 1.2 ms. This 20% variance may be acceptable, but if the required exposure time would be shorter, say 0.2 ms, the mechanical shutter could not be used because the actual exposure time would vary from 0 ms to 0.4 ms.
The problem becomes even worse when some new high dynamic range technologies are used. In these technologies, the exposure time is divided into two parts: long and short. So if the required exposure time is again 1 ms, the sensor would actually be exposed two times with, e.g., 0.8 ms and 0.2 ms exposure times. In this case, the mechanical shutter variance would be equal to the short exposure time, i.e., instead of the requested 0.2 ms short exposure time, the actual short exposure time would be between zero and 0.4 ms. This will obviously make the high dynamic range mode unusable.
In earlier implementations, the exposure time was selected so that the mechanical shutter variation was not too larger a portion of a total exposure time. The exposure time has been made longer by allowing less light to hit the sensor. This has been achieved using a filter in front of the lens or a variable iris, whose aperture becomes smaller if the requested exposure time becomes too short compared to its variance.
The problems in the earlier implementations include but are not limited to:                mechanical components, such as filter or variable iris, increase the cost and size the of the camera module.        very short exposure times are not possible: this may cause problems in some photographic situations, such as capturing a very fast movement.        